Thallium-201
Tl-201 · Thallium, Z = 81, A = 201
At 7.90e+15 Bq/g — 2.14e+5 Ci/g — a gigabecquerel of Tl-201 amounts to 127 ng, which is why activity rather than mass is how anyone states the quantity. Thallium-201 decays by electron capture with a half-life of 3.0420 days, falling to 20.29% of today's activity in a week and 9.9e-8% in three months.
At 0.0124 mGy·m²/(GBq·h) the air kerma rate constant is 6.2× less than Cs-137 and 25× less than Co-60, placing it 66 of 96 photon emitters in this dataset. 1 GBq at 1 m reads 0.0124 mGy/h, and 1 Ci at the same distance 0.457 mGy/h.
12 lines clear the 20 keV cutoff, but 5 of them carry 90% of the dose rate. The leading one is 70.8 keV at 32.0% of the total — its emission probability is 45.8%, which is also the highest.
A half-value layer of 0.224 mm in lead puts this in foil and thin sheet, with 1.47 mm needed if the material is steel; ten-fold attenuation comes at 0.804 mm of lead. At 1 GBq and a metre it is already under 20 µSv/h with nothing in the way. The tenth-value layer runs 3.6 times the half-value layer, not the 3.32 a single energy would give.
Half-life, specific activity and dose rate
| Half-life | 3.0420 days (2.628e+5 s) |
| Decay mode | electron capture |
| Specific activity | 7.90e+15 Bq/g (2.14e+5 Ci/g) |
| Air kerma rate constant Γ (δ = 20 keV) | 0.0124 mGy·m²/(GBq·h) |
| Dose rate, 1 GBq at 1 m | 0.0124 mGy/h |
| Dose rate, 1 Ci at 1 m | 0.457 mGy/h |
| Kerma-weighted mean photon energy | 90.77 keV |
12 lines above the cutoff, and what each contributes
2 further lines below the 20 keV cutoff, the highest at 11.78 keV and 42.2% emission probability in all, are excluded here and from Γ. Why the two columns rank differently.
| Energy (keV) | Emission probability (%) | Share of dose rate (%) |
|---|---|---|
| 70.82 | 45.8 | 32.00 |
| 68.89 | 27 | 18.77 |
| 167.43 | 10 | 15.92 |
| 81.28 | 20.1 | 14.56 |
| 80.32 | 15.5 | 11.12 |
| 82.48 | 4.55 | 3.34 |
| 135.34 | 2.605 | 3.21 |
| 30.60 | 0.258 | 0.43 |
| 32.19 | 0.263 | 0.40 |
| 165.88 | 0.147 | 0.23 |
| 26.27 | 0.0082 | 0.02 |
| 141.10 | 0.0026 | 0.00 |
0.224 mm of lead halves this spectrum
Solved numerically across all 12 lines, narrow beam. Why not one representative energy.
| Material | HVL (mm) | TVL (mm) | TVL / HVL |
|---|---|---|---|
| lead | 0.224 | 0.804 | 3.58 |
| tungsten | 0.0640 | 0.388 | 6.05 |
| iron | 1.47 | 6.42 | 4.36 |
| copper | 1.02 | 4.71 | 4.64 |
| concrete | 14.8 | 51.1 | 3.44 |
| water | 38.1 | 128 | 3.36 |
| aluminum | 12.6 | 43.6 | 3.47 |
A single energy would give 3.32. What a spread of energies does instead.
Activity over months and years
Ten half-lives is 30.4 days — a storage problem rather than a disposal one, with 7.2e-35% of today's activity still there after a year. The mean life 1/λ is 4.39 days.
| Elapsed | Fraction remaining |
|---|---|
| 1 half-life | 50.0 % |
| 2 half-lives | 25.0 % |
| 5 half-lives | 3.12 % |
| 10 half-lives | 0.0977 % |
| Time to fall to 10 % of today's activity | 10.1 days |
| Time to fall to 1 % | 20.2 days |
| Time to fall to 0.1 % | 30.3 days |
Limits of these dose rates
- 0.0124 mGy/h at a metre — bare point source, no capsule, no self-absorption.
- 0.224 mm of lead halves this spectrum, narrow beam, scatter not added back.
- Γ excludes 2 lines under 20 keV, carrying 42.2% of all emissions.
- What every sheet leaves out, internal dose included.
Gamma and decay calculators for Tl-201
Other Thallium nuclides: Tl-201m, Tl-204
Computed from the IAEA Nuclear Data Section — Livechart API (ENSDF) and the NIST X-Ray Mass Attenuation Coefficients. Derivations and citations.